The recording of moving images is becoming more and more popular. Cameras having image sensors with CCD technology (CCD: charged coupled devices) have provided a breakthrough in this field. In particular, such camera systems are becoming increasingly smaller and lighter. This has made it possible to install photo or video cameras in mobile telephones, so that mass use of video telephony, for example, for video conferencing, has come within reach. With the introduction of the code multiplexing method (UMTS), this application in the field of moblie radio communications received a new boost.
However, the practical implementation involves some difficulties. In order to record one's own face, mobile telephones are typically held with one hand and with the arm half extended. This position, especially in conjunction with the small size and low weight of the mobile telephone, makes it particularly difficult to hold the mobile telephone still so as to ensure video transmission with little blur. Especially during mobile phone video conferencing, it is a problem when the camera movements and the speaker movements are different and the image is therefore blurred. Even slight, often unconscious movements of the hand or of the head already cause noticeable shaking of the image to be transmitted. However, blurred video transmission significantly reduces the perceived quality.
Several shake compensation systems are already known from video camera applications.
Document EP 04 98 730 A2, for example, describes a video signal processing apparatus which can be used in a video camera in conjunction with a device for compensating for vibrations caused by the hand. This apparatus is provided with means that generate a motion vector representing an amount and direction of motion between consecutive frames of a video signal.
The described apparatus further has means capable of deciding whether or not the determined motion vector represents an unwanted movement of the camera. Also provided is a device for generating correction or control signals as a function of the determined motion vector to allow unwanted movement of the camera to be compensated for.
However, the motion vector determination is still in need of improvement in terms of the requirements for mobile radio communications and the recording of faces with their specific features.
U.S. Pat. No. 5,030,984 discloses a device which detects the motion in a scene by sampling the change in intensity of light of consecutive images at each pixel site in the image along with the change in intensity of light of a neighboring pixel site. These changes are then used to calculate both the displacement of the entire image and a directional degree of image change. Finally, a signal representing the total displacement is calculated and used, inter alia, to control the movement of a film in a direction in order to minimize blur in the recorded image.
Another document, EP 0 956 695 B1, discloses a motion sensor, which performs analysis on a plurality of images and determines a scene-motion parameter therefrom. The calculation unit, in this document also referred to as “motion sensor”, can be part of an image stabilization system. According to the description, the most practical technique is to analyze two sequential, successive fields or frames to determine the scene-motion parameter.
All of these systems operate fully electronically, so that, typically, part of the potential resolution is sacrificed for the sake of stabilization. This is because, in electronic image stabilization, it is common practice today to use unused edge pixels of the CCD image converter to detect movements of the video or photographer and to subsequently compensate for them mathematically. As in the case of many electronic aids, such as digital zoom or pixel interpolation, this results in a more or less significant loss in quality. This may not be perceived as too great a disadvantage in high-resolution systems.
But particularly in video telephones, the reasonably transmittable image resolution and repetition rate are significantly lower, in spite of the constantly increasing data transfer rates. Yet, it is especially in the case of mobile telephones that there are, on the one hand, extreme demands on size and, on the other hand, enormous cost pressure, so that the cheapest possible, low-resolution CCD chips are used. In this connection, typical resolutions of a video telephone may be in the range of VGA.
It is therefore desirable to have an image stabilization system that, other than digital image editing, substantially retains the resolution of the image recording sensor.
Optical image stabilization is mainly used in binoculars, video cameras and miniature camera lenses. The movements of the user are detected by so-called “gyroscopes”. Gyroscopes are devices capable of detecting motions about the horizontal and vertical axes. Once the camera or the lens system detects the direction in which the pick-up lens is moving, a special refractive lens, which is mostly supported in a liquid, is almost simultaneously moved in exactly the opposite direction, so that camera shake is largely compensated for.
Document DE 199 42 900 B4, for example, discloses a device for correcting image defects caused by camera shake. Three pairs of acceleration sensors are provided to detect camera shake in the horizontal and vertical directions as well as rotation. The correction signals supplied by the acceleration sensors are fed to multilayer piezoelectric actuators, which are connected to a CCD sensor. Using the multilayer piezoelectric actuators, the CCD sensor can be displaced in the X-direction and in the Y-direction, which disadvantageously allows only limited correction. On the other hand, the use of six acceleration sensors appears to be cumbersome, complex, and space-intensive.
To compensate for unwanted rotation of the camera body, there is also provided a rotary member, for example, in the form of a rotary shaft, which cooperates with a multilayer piezoelectric actuator. However, a rotary shaft involves a high degree of constructional complexity, is prone to failure, and, at best, poorly suited for applications with tight spatial constraints, such as in mobile telephones.
Disadvantageously, however, this technology does not provide for compensation for subject or object shake.
These systems, which have been developed for video cameras, which are large in size compared to a mobile telephone, can generally not be easily applied to a mobile telephone because of the extreme space constraints. Moreover, the stability requirements in mobile telephones are generally higher than in cameras, which are typically handled with more care and attention.